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More atmospheric carbon dioxide in the 1960s meant greenery flourished – but our photosynthesising friends have long had their fill. Kate Ravilious reports.

Trees and plants have had enough. For the past few decades they've obliged us by guzzling ever-greater amounts of atmospheric carbon dioxide every year – but now they've gone on a diet.

New data shows 'peak carbon', when vegetation consumed its largest carbon dioxide feast, occurred in 2006, and since then appetite has been decreasing.

“It's the first evidence that we are tipping over the edge potentially towards runaway or irreversible climate change,” says James Curran, former chief executive of the Scottish Environment Protection Agency and co-author of the study published in the journal Weather.

The news has come as a shock. Previous estimates indicated that peak carbon would not be reached until at least 2030.

Instead, the new data reveals that trees and plants are already 10 years beyond peak carbon. In 2014 alone, the shortfall in carbon absorption was equivalent to a year's worth of human-produced emissions from China.

“By next year the shortfall might equate to the emissions of China plus Australia, for example,” Curran explains.

“Every year it is getting a little bit worse.”

Carrying out the work in their own time with their own funds, Curran and his son Sam analysed the ups and downs in atmospheric carbon dioxide concentrations recorded at the Mauna Loa Observatory in Hawaii since 1958.

During northern hemisphere summer, carbon dioxide levels dip as fresh plant growth draws out carbon dioxide (the southern hemisphere is ocean dominated and so fails to balance this effect out).

The Mauna Loa data revealed carbon dioxide dips were deeper through the 1960s and early 1970s as northern hemisphere plants flourished in a rising carbon dioxide world.

But beyond the 1970s that rate slowed, reaching a peak in 2006 and declining thereafter.

“Some of the assumptions made in the study still need to be validated, but it does highlight that potentially the benefits of global carbon dioxide fertilisation are already behind us,” says Andreas Heinemeyer from the University of York in the UK, who was not involved in the study.

Exactly why plants and trees lost their appetite so soon is not yet known, but Curran believes it is likely linked to stresses associated with global warming to date such as drought, heat and fires.

This decline in plant's carbon dioxide appetite helps explain why atmospheric carbon dioxide levels have been rising faster than ever of late, despite our emissions more or less stabilising in recent years.

And as the plants' appetite continues to decline, the fight to tackle global warming will become even harder.

“There will come a point at which plants and trees stop soaking up any carbon dioxide and then the biosphere transitions into an emitter,” Curran says.

“Then – even if we stop all man-made emission – the planet itself goes on emitting and climate change is irreversible.”

It is a bleak outlook, and Curran believes only drastic action can wrestle global warming under control: “It suggests to me that we urgently need to get to grips with declining biodiversity across the globe, and consider radical new policies such as re-wilding large areas of landscape.”